The present invention relates to an electrohydraulic switching device adapted to issue electric control signals in response to pressure signals received thereby.
There is already known a device of this type including a first piston adapted to be acted upon by the input pressure and spring-loaded in opposition to the input pressure, and a second piston positively engageable with the first piston to activate an electric switching element, wherein the pistons are axially slidable against each other by a determined distance and a friction member engages the circumferential surface of the second piston to generate a switch hysteresis.
One construction of the switching device of this type consists essentially of two cylinder bores coaxially arranged in a housing, of which one is supplied with the pressure derived from a pressure accumulator. This cylinder bore accommodates a first piston acted upon by the force of a spring in opposition to the input pressure and extending with one of its ends into the second cylinder bore. An end wall is arranged between the cylinder bores, being sealed by a suitable sealing material to prevent the passage of pressurized fluid from the first to the second cylinder bore.
When pressure is supplied to the first cylinder bore, the first piston will be displaced against the force of the spring until the end of the first piston that is arranged in the second cylinder bore engages an abutment surface of the second piston to move the latter, at sufficient pressure in the first cylinder bore, in the direction of an electric switch to thereby cause the issuance of an electric switch signal.
Before the first piston engages the abutment surface of the second piston, a clearance has to be eliminated first. Once the second piston is in an actuating position with the accumulator pressure at its maximum, the pump charging the accumulator will be turned off. The accumulator pressure will be reduced on the withdrawal of fluid from the accumulator. With a reduction in the accumulator pressure, the first piston will be gradually urged into its position of rest by the spring, again performing a lost motion until it is in positive engagement with the second piston and thereby actuates the electric switch to energize the pump.
A spring-loaded friction member engages the circumferential surface of the second piston to hold the latter in the actuating position against the spring force of the contact pin of the electric switch, until the first and second pistons are frictionally engaged. In the switching device described, the upper and lower threshold values of the accumulator pressure are determined by the lost travel between the first and second piston and by the pressurized area of the first piston.
Because of its construction and mode of operation, the switching device described above may be used to control a fluid pump such that the pressure in the accumulator varies between two predetermined threshold or limiting values.
In switching devices of this type, it has been established, however, that a functional interrelationship exists between the actual limiting values of the accumulator pressure and the operating temperature or viscosity of the fluid, which is to be considered to constitute a disadvantage.